The present invention relates to optical filters, and in particular to dispersion-free optical filters for use as interleavers.
In dense wavelength division multiplexing (DWDM) optical telecommunications systems, it is advantageous to provide a very narrow width optical filter, which isolates individual transmission channels without distorting the information being transmitted. Accordingly, the optical filter must have a flat-top intensity response and no chromatic dispersion in the pass band. These two conditions guarantee that the filter will not distort the signal either in intensity or in phase.
The first condition, i.e. flat-top intensity response, is well known and has been achieved through various technologies, such as thin film filters, fiber Bragg gratings, coupled resonant cavities, and lattice filters. However, the second condition, i.e. no chromatic dispersion, (or equivalently having a linear phase response in the pass band) is harder to achieve for optical filters, especially when combined with the flat-top requirement. Resonant interleavers have an inherently high dispersion, and any dispersion compensation, which would only be partial, is very difficult to achieve,
It is an object of the present invention to disclose practical ways of realizing optical filters with substantially no chromatic dispersion, apart from the material induced dispersion, regardless of the intensity profile of the optical filter. Applications can be envisaged for any type of optical filter, including flat-top filters and interleavers.
Accordingly the present invention relates to a cascaded optical filter defined by transfer function HT(f) comprising:
a first optical filtering means defined by transfer function of H1(f); and
a second optical filtering means, optically coupled to the first optical filtering means, defined by transfer function H2(f);
wherein the transfer function of the second optical filtering means H2(f) complex conjugate of the transfer function of the first optical filtering means H1*(f);
whereby HT(f)=H1(f)xc3x97H2(f)=H1(f)xc3x97H1*(f).